/*
    TunesRemote+ - http://code.google.com/p/tunesremote-plus/
    
    Copyright (C) 2008 Jeffrey Sharkey, http://jsharkey.org/
    Copyright (C) 2010 TunesRemote+, http://code.google.com/p/tunesremote-plus/
    
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.
    
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
    
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
    
    The Initial Developer of the Original Code is Jeffrey Sharkey.
    Portions created by Jeffrey Sharkey are
    Copyright (C) 2008. Jeffrey Sharkey, http://jsharkey.org/
    All Rights Reserved.
 */
package org.tunesremote.util;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicInteger;

import android.os.Handler;
import android.os.Message;
import android.os.Process;

/**
 * <p>
 * UserTask enables proper and easy use of the UI thread. This class allows to
 * perform background operations and publish results on the UI thread without
 * having to manipulate threads and/or handlers.
 * </p>
 * <p>
 * A user task is defined by a computation that runs on a background thread and
 * whose result is published on the UI thread. A user task is defined by 3
 * generic types, called <code>Params</code>, <code>Progress</code> and
 * <code>Result</code>, and 4 steps, called <code>begin</code>,
 * <code>doInBackground</code>, <code>processProgress<code> and <code>end</code>
 * .
 * </p>
 * <h2>Usage</h2>
 * <p>
 * UserTask must be subclassed to be used. The subclass will override at least
 * one method ({@link #doInBackground(Object[])}), and most often will override
 * a second one ({@link #end(Object)}.)
 * </p>
 * <p>
 * Here is an example of subclassing:
 * </p>
 * 
 * <pre>
 * private class DownloadFilesTask extends UserTask&lt;URL, Integer, Long&gt; {
 *    public File doInBackground(URL... urls) {
 *       int count = urls.length;
 *       long totalSize = 0;
 *       for (int i = 0; i &lt; count; i++) {
 *          totalSize += Downloader.downloadFile(urls[i]);
 *          publishProgress((int) ((i / (float) count) * 100));
 *       }
 *    }
 * 
 *    public void processProgress(Integer... progress) {
 *       setProgressPercent(progress[0]);
 *    }
 * 
 *    public void end(Long result) {
 *       showDialog(&quot;Downloaded &quot; + result + &quot; bytes&quot;);
 *    }
 * }
 * </pre>
 * <p>
 * Once created, a task is executed very simply:
 * </p>
 * 
 * <pre>
 * new DownloadFilesTask().execute(new URL[] { ... });
 * </pre>
 * 
 * <h2>User task's generic types</h2>
 * <p>
 * The three types used by a user task are the following:
 * </p>
 * <ol>
 * <li><code>Params</code>, the type of the parameters sent to the task upon
 * execution.</li>
 * <li><code>Progress</code>, the type of the progress units published during
 * the background computation.</li>
 * <li><code>Result</code>, the type of the result of the background
 * computation.</li>
 * </ol>
 * <p>
 * Not all types are always used by a user task. To mark a type as unused,
 * simply use the type {@link Void}:
 * </p>
 * 
 * <pre>
 * private class MyTask extends UserTask<Void, Void, Void) { ... }
 * </pre>
 * 
 * <h2>The 4 steps</h2>
 * <p>
 * When a user task is executed, the task goes through 4 steps:
 * </p>
 * <ol>
 * <li>{@link #begin()}, invoked on the UI thread immediately after the task is
 * executed. This step is normally used to setup the task, for instance by
 * showing a progress bar in the user interface.</li>
 * <li>{@link #doInBackground(Object[])}, invoked on the background thread
 * immediately after {@link #begin()} finishes executing. This step is used to
 * perform background computation that can take a long time. The parameters of
 * the user task are passed to this step. The result of the computation must be
 * returned by this step and will be passed back to the last step. This step can
 * also use {@link #publishProgress(Object[])} to publish one or more units of
 * progress. These values are published on the UI thread, in the
 * {@link #processProgress(Object[])} step.</li>
 * <li>{@link #processProgress(Object[])}, invoked on the UI thread after a call
 * to {@link #publishProgress(Object[])}. The timing of the execution is
 * undefined. This method is used to display any form of progress in the user
 * interface while the background computation is still executing. For instance,
 * it can be used to animate a progress bar or show logs in a text field.</li>
 * <li>{@link #end(Object)}, invoked on the UI thread after the background
 * computation finishes. The result of the background computation is passed to
 * this step as a parameter.</li>
 * </ol>
 * <h2>Threading rules</h2>
 * <p>
 * There are a few threading rules that must be followed for this class to work
 * properly:
 * </p>
 * <ul>
 * <li>The task instance must be created on the UI thread.</li>
 * <li>{@link #execute(Object[])} must be invoked on the UI thread.</li>
 * <li>Do not call {@link #begin()}, {@link #end(Object)},
 * {@link #doInBackground(Object[])}, {@link #processProgress(Object[])}
 * manually.</li>
 * <li>The task can be executed only once (an exception will be thrown if a
 * second execution is attempted.)</li>
 * </ul>
 */
public abstract class UserTask<Params, Progress, Result> {
   private static final String LOG_TAG = "UserTask";

   private static final int CORE_POOL_SIZE = 4;
   private static final int MAXIMUM_POOL_SIZE = 50;
   private static final int KEEP_ALIVE = 10;

   private static final BlockingQueue<Runnable> sWorkQueue = new LinkedBlockingQueue<Runnable>(MAXIMUM_POOL_SIZE);

   private static final ThreadFactory sThreadFactory = new ThreadFactory() {
      private final AtomicInteger mCount = new AtomicInteger(1);

      public Thread newThread(Runnable r) {
         final Thread thread = new Thread(r, "UserTask #" + mCount.getAndIncrement());
         Process.setThreadPriority(Process.THREAD_PRIORITY_LOWEST);
         return thread;
      }
   };

   private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE,
            KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);

   private static final int MESSAGE_POST_RESULT = 0x1;
   private static final int MESSAGE_POST_PROGRESS = 0x2;

   private static InternalHandler sHandler;

   private final WorkerRunnable<Params, Result> mWorker;
   private final FutureTask<Result> mFuture;

   private volatile Status mStatus = Status.PENDING;

   /**
    * Indicates the current status of the task. Each status will be set only
    * once during the lifetime of a task.
    */
   public enum Status {
      /**
       * Indicates that the task has not been executed yet.
       */
      PENDING,
      /**
       * Indicates that the task is running.
       */
      RUNNING,
      /**
       * Indicates that {@link UserTask#end(Object)} has finished.
       */
      FINISHED,
   }

   /**
    * Creates a new user task. This constructor must be invoked on the UI
    * thread.
    */
   public UserTask() {
      if (sHandler == null) {
         sHandler = new InternalHandler();
      }

      mWorker = new WorkerRunnable<Params, Result>() {
         public Result call() throws Exception {
            return doInBackground(mParams);
         }
      };

      mFuture = new FutureTask<Result>(mWorker) {
         @Override
         protected void done() {
            Result result = null;
            try {
               result = get();
            } catch (InterruptedException e) {
               android.util.Log.w(LOG_TAG, e);
            } catch (ExecutionException e) {
               throw new RuntimeException("An error occured while executing doInBackground()", e.getCause());
            } catch (CancellationException e) {
               return;
            } catch (Throwable t) {
               throw new RuntimeException("An error occured while executing " + "doInBackground()", t);
            }

            final Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT, new UserTaskResult<Result>(
                     UserTask.this, result));
            message.sendToTarget();
         }
      };
   }

   /**
    * Returns the current status of this task.
    * @return The current status.
    */
   public final Status getStatus() {
      return mStatus;
   }

   /**
    * Override this method to perform a computation on a background thread. The
    * specified parameters are the parameters passed to
    * {@link #execute(Object[])} by the caller of this task. This method can
    * call {@link #publishProgress(Object[])} to publish updates on the UI
    * thread.
    * @params params The parameters of the task.
    * @return A result, defined by the subclass of this task.
    * @see #begin()
    * @see #end(Object)
    * @see #publishProgress(Object[])
    */
   public abstract Result doInBackground(Params... params);

   /**
    * Runs on the UI thread before {@link #doInBackground(Object[])}.
    * @see #end(Object)
    * @see #doInBackground(Object[])
    */
   public void begin() {
   }

   /**
    * Runs on the UI thread after {@link #doInBackground(Object[])}. The
    * specified result is the value returned by
    * {@link #doInBackground(Object[])} or null if the task was cancelled or an
    * exception occured.
    * @see #begin()
    * @see #doInBackground(Object[])
    */
   public void end(Result result) {
   }

   /**
    * Runs on the UI thread after {@link #publishProgress(Object[])} is invoked.
    * The specified values are the values passed to
    * {@link #publishProgress(Object[])}.
    * @see #publishProgress(Object[])
    * @see #doInBackground(Object[])
    */
   public void processProgress(Progress... values) {
   }

   /**
    * Returns <tt>true</tt> if this task was cancelled before it completed
    * normally.
    * @return <tt>true</tt> if task was cancelled before it completed
    * @see #cancel(boolean)
    */
   public final boolean isCancelled() {
      return mFuture.isCancelled();
   }

   /**
    * Attempts to cancel execution of this task. This attempt will fail if the
    * task has already completed, already been cancelled, or could not be
    * cancelled for some other reason. If successful, and this task has not
    * started when <tt>cancel</tt> is called, this task should never run. If the
    * task has already started, then the <tt>mayInterruptIfRunning</tt>
    * parameter determines whether the thread executing this task should be
    * interrupted in an attempt to stop the task.
    * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
    *           task should be interrupted; otherwise, in-progress tasks are
    *           allowed to complete.
    * @return <tt>false</tt> if the task could not be cancelled, typically
    *         because it has already completed normally; <tt>true</tt> otherwise
    * @see #isCancelled()
    */
   public final boolean cancel(boolean mayInterruptIfRunning) {
      return mFuture.cancel(mayInterruptIfRunning);
   }

   /**
    * Waits if necessary for the computation to complete, and then retrieves its
    * result.
    * @return The computed result.
    * @throws CancellationException If the computation was cancelled.
    * @throws ExecutionException If the computation threw an exception.
    * @throws InterruptedException If the current thread was interrupted while
    *            waiting.
    */
   public final Result get() throws InterruptedException, ExecutionException {
      return mFuture.get();
   }

   /**
    * Waits if necessary for at most the given time for the computation to
    * complete, and then retrieves its result.
    * @return The computed result.
    * @throws CancellationException If the computation was cancelled.
    * @throws ExecutionException If the computation threw an exception.
    * @throws InterruptedException If the current thread was interrupted while
    *            waiting.
    * @throws TimeoutException If the wait timed out.
    */
   public final Result get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException,
            TimeoutException {
      return mFuture.get(timeout, unit);
   }

   /**
    * Executes the task with the specified parameters. The task returns itself
    * (this) so that the caller can keep a reference to it. This method must be
    * invoked on the UI thread.
    * @params params The parameters of the task.
    * @return This instance of UserTask.
    * @throws IllegalStateException If {@link #getStatus()} returns either
    *            {@link com.google.android.photostream.UserTask.Status#RUNNING}
    *            or
    *            {@link com.google.android.photostream.UserTask.Status#FINISHED}
    *            .
    */
   public final UserTask<Params, Progress, Result> execute(Params... params) {
      if (mStatus != Status.PENDING) {
         switch (mStatus) {
         case RUNNING:
            throw new IllegalStateException("Cannot execute task:" + " the task is already running.");
         case FINISHED:
            throw new IllegalStateException("Cannot execute task:" + " the task has already been executed "
                     + "(a task can be executed only once)");
         }
      }

      mStatus = Status.RUNNING;

      begin();

      mWorker.mParams = params;
      sExecutor.execute(mFuture);

      return this;
   }

   /**
    * This method can be invoked from {@link #doInBackground(Object[])} to
    * publish updates on the UI thread while the background computation is still
    * running. Each call to this method will trigger the execution of
    * {@link #processProgress(Object[])} on the UI thread.
    * @params values The progress values to update the UI with.
    * @see #processProgress(Object[])
    * @see #doInBackground(Object[])
    */
   protected final void publishProgress(Progress... values) {
      sHandler.obtainMessage(MESSAGE_POST_PROGRESS, new UserTaskResult<Progress>(this, values)).sendToTarget();
   }

   private void finish(Result result) {
      end(result);
      mStatus = Status.FINISHED;
   }

   private static class InternalHandler extends Handler {
      @SuppressWarnings( { "unchecked" })
      @Override
      public void handleMessage(Message msg) {
         UserTaskResult result = (UserTaskResult) msg.obj;
         switch (msg.what) {
         case MESSAGE_POST_RESULT:
            // There is only one result
            result.mTask.finish(result.mData[0]);
            break;
         case MESSAGE_POST_PROGRESS:
            result.mTask.processProgress(result.mData);
            break;
         }
      }
   }

   private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
      Params[] mParams;
   }

   private static class UserTaskResult<Data> {
      final UserTask mTask;
      final Data[] mData;

      UserTaskResult(UserTask task, Data... data) {
         mTask = task;
         mData = data;
      }
   }
}
